[Amps] MOV's

[jljarvis]

Steve, Richard:

I was looking at MOV's used in surge suppressors on telco and 
power drops.  We wre zapping them with a measured charge, per
IEEE, and observing the resulting waveform.  

One of the uncomfortable things is, they sometimes fail open..
so you don't know they've failed.  

Jim

-----Original Message-----
From: Steve Katz [mailto:stevek@jmr.com]
Sent: Friday, March 15, 2002 1:27 PM
To: 'Richard'; jljarvis; AMPS
Subject: RE: [Amps] MOV's


Note abt MOVs:

One thing that unfortunately does not appear in the mfr's data sheets, or in
any parametric/dynamic curves, is the definition of how the MOV crystals are
arranged and how many joules it takes to fuse each grain.

Traditional MOVs (all that I've seen, anyway) are doped grains of metal
oxides, primarily zinc oxide, that are placed in series, or in
series-parallel combinations, to achieve the BV and J rating desired.  A
large surge can fuse one or two grains and not the rest.  That results in a
lower BV.  This isn't an open nor a short, it's just a modified MOV which
has a lower BV than it originally had.  Unless you're in the habit of
removing them to measure this characteristic after each surge, it's
impossible to know what's happened within the MOV.

As the BV is reduced and gets close to the actual operating voltage, leakage
current within the MOV causes heating (of course), and as the MOV grains get
hotter, their BV drifts more until eventually -- bingo!  With luck, what you
normally have is simply a blown fuse and an MOV you need to replace.

WB2WIK/6

"Each success only buys an admission ticket to a more difficult problem." --
Henry Kissinger

> -----Original Message-----
> From:	Richard [SMTP:2@mail.vcnet.com]
> Sent:	Friday, March 15, 2002 9:42 AM
> To:	jljarvis;  AMPS
> Subject:	Re: [Amps] MOV's
> 
> 
> 
> >
> >Careful, here.....MOV's shift characteristics with dosage, and
> >may only be good for a few shots, depending on the charge it
> >has to absorb, per shot.
> >
> EUR  MOVs come in a wide range of repetitive peak-current abilities.  The 
> little guys have thin wire leads and the biguns have strap leads.   
> Picking the right one for the job at hand is not difficult.  When in 
> doubt, $pring for the next size up. 
> 
> ><snip>
> >>a MOV across the contacts cures the  problem better than a vacuum-
> >>>relay since the reverse EMF still needs to be suppressed to prevent
> >>>damage elsewhere.
> >>
> >>The problem is that because it's DC, even with no inductive load,
> >
> >EUR  If wires are used to connect a (pretend) purely resistive load, the
> >load always looks iinductive.  And when the circuit opens, a reverse EMF
> >at the opening contacts is guaranteed.  Good engineering practice is to
> >suppress reverse EMFs with a MOV or bi-lateral Si transient suppressor
> >diode.
> ><snip>
> >
> >I did some work with MOV transient suppressors in a previous lifetime.
> >If you zap them in opposite directions more or less equally, they can
> >last a while.  Zap them in one direction, like reverse EMF, and they
> >will become increasingly less useful.
> >
> EUR  When in doubt, read the fine print in the mfg specs.  
> 
> >Jim N2EA
> >
> >PS:  TWA's (retired) senior 747 check pilot at the time of the 'center 
> >fuel tank' explosion, still believes it was friendly fire that downed the
> 
> aircraft.
> >
> EUR  As do many govamint conspiracy experts who have seen black
> helicopters 
> and Russky tanks in the corn fields of Kansas. 
> 
> cheers, Jim
> 
> -  R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K, 
> www.vcnet.com/measures.  
> end
> 
> _______________________________________________
> Amps mailing list
> Amps@contesting.com
> http://lists.contesting.com/mailman/listinfo/amps